Improved measurements of pollution emissions by automobiles have become increasingly important as the overall emissions have decreased, since more precision is required to accurately gauge the various pollutants. Instrumentation is available for making accurate measurements in fixed locations, such as garages, where the vehicles are stationary. However, such measurements do not represent vehicle emissions under actual operating conditions; also, vehicle emission standards relying on such periodic measurements can be circumvented.
It is therefore desirable to analyze emissions of individual vehicles "on the fly" so to speak, as they pass measurement sites in the course of normal travel over highways or city streets. Several remote sensing cross-road instruments capable of instantaneously measuring pollutants emitted by passing vehicles have been described. These devices work by directing infrared or ultraviolet radiation across the roadway. Passing cars leave exhaust plumes in the radiation paths and the molecules in the exhaust absorb some of the radiation. The amount of radiation absorbed can be correlated with the column density of the molecular absorber. The column density of carbon dioxide is measured and the emission indices of pollutants are determined as ratios with respect to carbon dioxide. This reference molecule allows meaningful pollutant emission measurements to be made without knowledge of the precise location or extent of dilution of the exhaust plume.
Although these devices have been adequate for monitoring the historically high concentrations of carbon monoxide which they were first developed to measure, they suffer from a relatively low signal-to-noise ratio and they therefore do not provide the desired accuracy for measurement of the concentrations of the various other pollutants present at much lower levels. Specifically, the radiation sources are incoherent, relatively broad-band lamps. The filters used to preferentially pass the absorption frequencies of the various constituents to the detectors pass relative broad-bands of frequencies and the detectors thus receive inputs that are affected by a number of factors in addition to the monitored absorptions. In addition, the broad emitting area of incoherent sources limits the distance that light from these sources can propagate without significant spreading. The spreading over distance traveled of light from incoherent sources can be compensated for with larger beam-forming and collection optics, but such optics greatly increases the cost and complexity of these instruments. Also, these devices cannot calculate the absolute column density of a pollutant and so require frequent field calibration using expensive analyzed gas samples in order to operate in a dependable manner.